Polymeric thick film inks consist of particles containing conductive materials dispersed in an organic vehicle or medium containing volatile solvent and a polymeric resin. After screen-printing, the composition is dried typically by heating at temperatures of up to 150° C. where the organic solvent is dried-off.
The conductive material is responsible for imparting to the thick film material the desired level of resistivity. This is normally limited to less than 0.02 Ohm/sq/mil. The conductive particles typically consist of silver metal for high conductivity and good resistance to oxidation and can be found in flake or non-flake morphologies.
After drying, the polymeric resin's primary function is to bind together the conductive particles to form an electrically conductive circuit pattern. Additionally, the binder system is required to impart the necessary adhesion to the desired substrate. In the case of flexible substrates which may or may not be surface treated, thermoplastic binder systems are typically employed. In general, these consist of polyesters, acrylics, vinyl or polyurethane polymers and may be combined to obtain optimum properties.
Furthermore, the resinous component is also responsible for providing the conductor composition with the required surface hardness, resistance to environmental changes and flexibility.
Historically, linear polyester, acrylic or vinyl copolymer-based resins have been typically been used as the polymeric binder for general purpose membrane touch switch (MTS) conductor pastes. Other types of resins, such as polyurethane resins have also been utilized in membrane touch switch applications.
U.S. Pat. No. 4,595,605 discloses conductive compositions that are solderable and flexible and that can be bonded directly to substrates. These compositions are made from a combination of silver exclusively in the form of flake and vinyl chloride/vinyl acetate copolymer.
U.S. Pat. No. 4,371,459 teaches a screen printable conductor composition useful in membrane touch switch compositions that is flexible comprising (a) a conductive phase containing silver and base metal powders dispersed in a solution of (b) a multipolymer prepared by copolymerization of vinyl acetate, vinyl chloride, and an ethylenically unsaturated dicarboxylic acid and a linear aromatic polyester resin dissolved in (c) volatile nonhydrocarbon solvent.
U.S. Pat. No. 5,653,918 to Towlson discloses a highly flexible and mechanically robust screen-printable conductor composition containing (a) a conductive phase comprising Ag, Au, Cu, Ni, Pd, Pt, C or graphite and mixtures thereof dispersed in a polymer solution containing (b) a terpolymer of polyvinyl acetate, vinyl chloride and a polar component dissolved in a volatile solvent.
Although the above-listed patents cite improvements in adhesion of the composition to the desired substrate and durability, none of the listed patents appear to improve overall lifetime of the touch switch and control of silver migration. The present invention provides a superior thick film conductive composition which increases MTS lifetime by decreasing metal conductor migration.